It has been a common problem that posts, especially the major European Posts, have adopted the color blue for their franking inks. This decision was due to the larger absorbance range of the blue in the visible spectrum as opposed to red inks. Most of the detectors in the available postal scanners are optimized for OCR mainly in black and, therefore, will be more suitable to detect blue than red. Since various European Posts have different and sometimes contradictory requirements, there is a need for a single ink that will satisfy all European and possibly other postal requirements.
One important requirement which is contradictory to others, is for extremely high contrast on porous substrates (>70%) while keeping the color blue with certain limits for the color parameters contradicting the high contrast requirement. Blue does not inherently exhibit high contrast, and the formulation modifications necessary to achieve a blue coloration on some substrates will not necessarily work on others.
Other contradictory parameters working against the identification of a single blue postal ink are the need durability in terms of waterfastness along with high contrast (high loading of the colorant) and reliability (decap time, purging). An important attribute of inkjet ink is “decap” time, i.e., the time a nozzle can remain dormant and then still fire a drop without misdirection or loss of velocity. Decap is to some extent affected by ink vehicle evaporation which leaves behind a deposit of nonvolatile ingredients that are detrimental to jetting performance. In other words, to achieve waterfastness and high contrast, it may be necessary to provide high concentrations of pigment and/or dyes, and this can adversely affect purging and decap properties.
There is a clear need for a blue postal ink for inkjet printing that is capable of providing durable, high contrast, machine-readable images, especially those containing bar code information, on a broad range of substrates while meeting a diverse array of regulatory requirements.